The present invention relates to an improved process for purifying lead bullion, and, more particularly, to a process which uses sodium for causing beneficial separations in crude lead bullion in which the sulfur content is less than about 1.3% by weight. The improvement comprises adding iron or an iron-containing reagent to the molten bullion whereby arsenic can be concentrated in the speiss phase.
U.S. Pat. No. 4,404,026 describes a method for treating and separating lead from relatively impure crude lead bullion and the matte and speiss phases which coexist therein. The process is accomplished by adding chemical reagents, e.g., sodium-containing reagents, to the molten bullion followed by a controlled solidification of the blast furnace mixture to substantially separate a matte and speiss phase from a lead bullion phase, thereby eliminating the need for dross reverberatory furnace separation. Separation of the lead with acceptable levels of residual impurities, e.g., arsenic levels, tends to become more difficult when the crude lead bullion is more pure, i.e., when it contains no more than a small amount of sulfur, e.g., less than about 1.3% by weight.
When cleaner, i.e., more pure, crude bullions are contacted with molten sodium, the phase and component separations, described, for example, in U.S. Pat. No. 4,404,026, can be constrained by a lack of suitable reagent for the sodium to reduce and a minimal production of non-bullion components which make isolation and recovery of the respective phases difficult. Undesirable components can be retained in the equilibrium phases, and the bullion may have high residual arsenic, or the ratio of copper to lead in the matte phase, for example, may be unsatisfactory.